Balsting agent and process for the production thereof



United States Patent 3,155,466 BLASTING AGENT AND PRGfiESS FOR THEPRGDUCTIUN THEREUF Kurt Griitter, Choindez, and Heinrich Starnptii,Courrendlin, Switzerland, assignors to Gesellschaft der Ludw. vonRolischen Eisenwerke Aktiengeseilschat't,

Gerlatingen, Switzerland, a Swiss company No Drawing. Filed Sept. 9,1959, Ser. No. 838,851 -Clahns priority, application Switzerland, Sept.11, 1958,

63,828 3 Claims. (Cl. 51-307) This invention relates to blasting agents.More particularly, it relates to a new blasting agent which consists ofslag; and to a method of producing said new blasting agent.

In industry, so-called blasting agents are usually employed to cleanmetallic surfaces, more especially to prepare them to receive a colouror metal coating, the blasting agents being projected on to the surfacesat high speeds and thus treating them in the desired manner.

Generally speaking, it is required of a blasting agent that it may beused for as long as possible, that is to say, that it can be used anumber of times without losing its properties, and, in addition, theblasting agent must be inexpensive. Blasting agents presently employedare: steel grit, moulding sand, corundum and various types of quartzsand, such as pure quartz sand or river sand and the like. All presentblasting agents are usually attended by certain disadvantages. Blastingagents consisting of cast iron or steel hammer the surface in suchmanner that the surface becomes too rough, which increases the danger ofunder-rusting in the case of spray metallization and painting. Very goodproperties are possessed by quartz sand, which, on the one hand is oflow cost and on the other is a relatively wear-resistant material whichpermits several applications, that is to say, it can be used severaltimes for blasting surfaces. However, this blasting agent, which is atpresent generally employed, has an outstanding disadvantage of anotherkind, which causes more and more opposition to its use. It has beenfound that the presence of quartz leads to silicosis, so thatpractically all of the workmen engaged in blasting installation areexposed to the danger of silicosis. Both the insurance companies andfactory inspection authorities are endeavouring to ban this dangeroussubstance from production processes and, thus, to eliminate such dangerto the workers.

Accordingly, it is an object of the present invention to provide for ablasting agent which is inexpensive, as are the commonly used blastingagents, such as quartz sand, and which is also free from the danger ofcausing silicosis in the lungs of the workmen.

An extremely valuable agent having optimum proper ties is granulatedcorundurn. its cost of production, however, is so high that, here again,general use is found to be impracticable.

It is another object of the invention to provide for a blasting agenthaving properties similar to those of corundum.

It is in itself obvious that, in the search for a suitable blastingagent, the materials first considered were waste products, such as slagand the like, since they were obtainable at a low cost. Furnace slagsexist which contain no free silica (quartz) and, therefore, give no riseto danger of silicosis. When such slags are granulated,

3,155,466 Patented Nov. 3, 1964 for example by quenching with water, asubstance is formed which at first sight appears to be readily usable asa blasting agent. This slag has a hardness of about 7, but is subject tointernal stresses. Consequently, the individual grain, when projected onto the metal surface, rapidly disintegrates into extremely fineconstituents and, thus, cannot be used.

It is another object of the invention to provide for a blasting agentusing furnace slags.

We have now found that a granulated, surface-sintered slag is a suitableblasting agent. By the term surfacesintered, we understand the followingphenomena: if ordinary hot slag is cooled at a relatively rapid rate, aplurality of granules result, having fissures which are caused by thissudden cooling. These granules would, under the impact of mechanicalstress, disintegrate along these fissures into smaller particles. Inaddition, the above-mentioned internal stresses render this materialpractically worthless as a blasting agent. We have now found that, by asuitable treatment of the slag, we are able to eliminate both fissuresand internal stress. This treatment consists in subjecting the particlesto the iniluence of heat, such that, by softening or sintering thesurface of the particles, with no merging of the surfaces of differentparticles, the surface fissures are closed. In this process, whereby thefissures are closed and the internal stresses are resolved, the granulesacquire very surprising properties, substantially similar to those ofcorundurn, in regard to wear and resistance, as well as efiectiveness asa blasting agent. The sintering temperature and the sintering period,which must be such as to cause closure of the fissures, must, naturally,be adapted to the chemical composition of the slag and the size of thegrains. These two conditions, sintering temperature and period, aredependent upon whether the heat treatment is eifected by radiant heatingor by heat transfer.

A preferred form of the blasting agent, according to the invention,consists of a slag having substantially the following composition hasproved excellently suitable:

Percent SiO 25-45 CaO-l-MgO 35-60 A1 0 8-25 And the ratio valueP=CaO/SiO -=0.8-2.

SiO 33-35 CaO-l-MgO 43-48 A1 0 15-20 With a ratio of CaOzSiO rangingfrom about 1.2-1.4.

In order that the invention may be better understood, the followingexamples are given:

I. A typical slag from a blast furnace for the processing of iron orewas granulated by pouring the slag in an essentially liquid state,having a temperature of about 1400 C., in water of a temperature ofabout 15 C.

:3 =3 The granular slag, with a grain size of up to about 3 mm., and aweight per unit volume of 1.4 kilograms per dm. and a hardness number,in terms of Mohs scale, of about 7 resulted.

The chemical composition of the slag, before and after granulation, was:

Percent CnO 43.6

SiO 33.8 A1 16.5 MgO 3.1 FeO 0.9

And traces of manganese, chrome, phosphor, etc.

A portion of the granulated material was heated by radiation in afurnace of the vaulted type (Borell furnace) for a period of about onehour. The temperature of the furnace ranged from 920940 C. The resultinggranulated material was cooled for a period of ten minutes, and showed amarked change in appearance as compared with the granulated slag beforeheating. The starting material had a light grey to greenish color and aglassy appearance. After heating, the material was dark grey to blackand had a dull appearance.

II. The procedure of Example I was repeated, with the exception that theheating for obtaining the surface-sintering of the slag was carried outin a rotary kiln at a temperature ranging from 950970 C. for a period ofabout five minutes. The resulting surfacesintered granulated slag was,in appearance and elfectiveness, as a blasting agent, equivalent to theproduct of Example I. The latter attribute shall be discussed in thefollowing example.

III. The slag resulting from Example H was tested as to itseffectiveness as a blasting agent. The granulated material was sievedand the fraction with the average size ranging from 1.0-1.5 mm. was usedin a blasting apparatus, having a nozzle with an inner diameter of sixmillimeters and a working pressure of 4 atmospheres. The blastingmaterial was blasted with this apparatus upon sheet-iron, with adistance of 5 cm. between nozzle and sheet-iron. In the following tablethe stability of the blasting agent was tested by running a given samplea number of times (3, 6, 9, 12) through the blasting process. Thenumbers in columns 2-5 give the percentage of the granulated materialretaining its original grain size.

The comparison of the values in columns 3 and 5 demonstrates that theproperties of the surface-sintered slag compare with the properties ofcorundum. The values of the slag, according to Examples I, gave the samevalues as the slag resulting from Example II, with the margin of errorof the measurement of the grain size.

It is to be noted that the values given in Examples I and II are notmeant to limit the invention. Any slag, with the above composition, andan appropriate degree of hardness, will give, after appropriatesintering of the surface, a suitable blasting material. Neither thetemperature values nor the period of heating are critical. The optimaldegree of heating for a given slag with a given size of the granules, isthe one which provides for complete closing of the fissures withoutmerging of the granules with each other. The optimal period of heatingis the shortest period of heating which, at a given temperature, isnecessary for attaining the closure of the fissures orsurface-sintering.

Adaptation of the conditions of treatment for a given slag will beevident to anyone experienced in the arts and using the above-specifiedteaching.

Various changes and modifications may be made without departing from thespirit and scope of the present invention, and it is intended that suchobvious changes and modifications be embraced by the annexed claims.

Having thus described the invention, what is claimed as new and desiredto be secured by Letters Patent, is:

l. A method of preparing a blasting agent from slag granules havingsurface fissures, which comprises reheating solidified fissured slaggranules to a temperature and for a period of time sufiicient to causesintering of a relatively thin surface layer of the slag granulesthereby to fuse and close said fissures but insutficient to causesintering of the interior of the slag granules and substantialagglomeration of adjacent granules and thereafter slowly cooling saidgranules to avoid internal stress.

2. A blasting agent comprising sintered slag particles.

3. A method as claimed in claim 1, wherein said cooling is effectedwithin about ten minutes.

discrete surface- References Cited in the file of this patent UNITEDSTATES PATENTS 1,763,937 Stievater June 17, 1930 1,980,432 Power Nov.13, 1934 2,038,251 Vogt Apr. 21, 1936 2,461,011 Taylor et al. Feb. 8,1949 2,532,548 Heide Dec. 5, 1950 2,533,633 Schott Dec. 12, 19502,610,922 Beck Sept. 16, 1952 3,054,139 Bartholomew et al. Sept. 18,1962 FOREIGN PATENTS 667,016 Great Britain Feb. 20, 1952 OTHERREFERENCES The Canadian Mining and Metallurgical Bulletin for March1954, Montreal, pages -169.

1. A METHOD OF PREPARING A BLASTING AGENT FROM SLAG GRANULES HAVING SURFACE FISSURES, WHICH COMPRISES REHEATING SOLIDIFIED FISSURED SLAG GRANULES TO A TEMPERATURE AND FOR A PERIOD OF TIME SUFFICENT TO CAUSE SINTERING OF A RELATIVELY THIN SURFACE LAYER OF THE SLAG GRANULES THEREBY TO FUSE AND CLOSE SAID FISSURES BUT INSUFFICIENT TO CAUSE SINTERING OF THE INTERIOR OF THE SLAG GRANULES AND SUBSTANTIAL AGGLOMERATION OF ADJACENT GRANULES AND THEREAFTER SLOWLY COOLING SAID GRANULES TO AVOID INTERNAL STRESS.
 2. A BLASTING AGENT COMPRISING DISCRETE SURFACESINTERED SLAG PARTICLES. 